Variations in rainfall and storm intensity over a broad swath of the Southern Hemisphere follow a pattern that repeats every 20 to 30 days. The pattern is the first regular atmospheric oscillation found outside the tropics and could help scientists forecast weather and climate changes in the region.

Weather is notoriously chaotic; even the best forecasts are rarely accurate more than a week or two out. In the last century, however, long-term climate patterns have emerged from seemingly disordered data. Most famously, El Niño is a cycle of atmospheric energy over the tropical Pacific Ocean that repeats every two to seven years; the pattern, known since the early 1900s, can cause floods and droughts around the world. More recently, in 1994, researchers found the Madden-Julian Oscillation, which sends storms churning eastward across the tropics on a 30- to 60-day cycle. Until now scientists had not found similar atmospheric oscillations in higher latitudes.

In the new study, David Thompson and Elizabeth Barnes of Colorado State University in Fort Collins looked for an atmospheric oscillation in more than 30 years of satellite and weather data from the Southern Hemisphere. The researchers report in the Feb. 7 Science that between 30° and 70° S latitude, a zone that extends from southern Africa and Australia to the Antarctic coast, storm energy and rainfall are not distributed randomly. Rather, the variables cycle up and down every 20 to 30 days. As a result, if rainfall and storminess over the southern midlatitudes are above average today, they will more likely be below average in roughly two weeks.

Thompson and Barnes then analyzed computer simulations of weather and climate. The team found that both simple and sophisticated simulations reproduce the pattern they had teased out of satellite data. Based on such models, the scientists think the oscillation occurs because the sun heats the tropics more than the poles. This differential heating causes a temperature imbalance to build until storms emerge and wipe it out, Barnes says; the entire process seems to take between 20 and 30 days.

Scientists have long known that storms build through this mechanism, but no one’s ever looked for a repeating pattern in the Southern Hemisphere, Barnes says.

“It’s really a comprehensive and complete study,” says Qinghua Ding, an atmospheric scientist at the University of Washington in Seattle. Finding the oscillation in both satellite data and computer simulations, he says, “makes it a more robust result.”

Meteorologist Steven Feldstein of Pennsylvania State University in University Park calls the discovery “a nice, interesting surprise.” He says the team’s explanation for the oscillation is convincing and thinks that with further study the pattern could help meteorologists forecast weather in the Southern Hemisphere.